Implants drops are based on XP events, are random and higher awared XP have a higher chance for an implant drop.
Fact is: the current drop tuning is designed to allow average players (by raw, non-boosted xp gain,
across all characters) to be able to maintain even tier 3 implants with drops alone. Check it out
for yourself. The loot system is more complex than just "kills per implant drop" or "xp per implant
drop", it's weighted and favors large xp events (base captures, ribbons, alerts) for better drops.
No one has scratched the surface of the drop system on pts yet. - Higby (PS2 Creative Director)
Initially there were caps in place for drop rates but they got removed at 21.05.2014.
There are not more information available about the inner workings of the drop system.